Abstract
The effects of frequency and temperature- and strain-waveform on thermal fatigue strength were examined by conducting out-of-phase and in-phase thermal fatigue tests with three kinds of temperature-waveforms (fast heating and fast cooling, slow heating and slow cooling, slow heating and fast cooling) under temperature cycling between 350-650°C and by isothermal low-cycle fatigue tests at 650°C under cyclic frequencies of 0.5∼0.039cpm. The following results were obtained.
(1) The effect of frequency on fatigue life in out-of-phase thermal fatigue was as small as in isothermal low-cycle fatigue, whereas in in-phase thermal fatigue it was much greater and the fatigue life reduction was more remarkable in low frequency.
(2) The effect of temperature-and strain-waveform on thermal fatigue life was still smaller than the effect of frequency on out-of-phase thermal fatigue life.
(3) The fracture mode of out-of-phase thermal fatigue was the transgranular type even at the lowest frequency tested, but that of in-phase thermal fatigue changed to the intergranular type at low frequency.
(4) Out-of-phase and in-phase thermal fatigue data obtained at the test condition of such an extremely low frequency as 0.039cpm were found to coincide well with ΔεPC-NPC andΔεCP-NCP relations, respectively.
